1. Field of the Invention
This invention relates to materials for shielding against electromagnetic radiation and a shielded room and, more specifically, to shielding material capable of shielding against electric fields and electromagnetic radiations of both high and low frequency as well as such radiations of both low and high energy level and a room shielded therewith.
2. Brief Description of the Prior Art
The problem of shielding against or attenuating electromagnetic radiations of the type generally known as interference has been well known and documented in the literature for many years. Such interference can be of many types and can have a narrow or wide frequency range, such as, for example, low frequency (up to about 10K Hertz) of the type emanating from an electric light bulb (60 Hertz), high frequency (greater than about 10K Hertz) of the type emanating from a computer or the like or combinations thereof. Such interference can also have high energy levels or low energy levels. Many different procedures for shielding against such radiation have been developed over the years with varying degrees of success. No single shielding material is fully effective against all types of interference of the type set forth above. It therefore is and has long been and will continue to be a need of the art to provide ever better shielding materials to shield against all of the types of interference enumerated hereinabove.
Shielding of interference from electromagnetic radiation has been found useful if not necessary in many environments, such as, for example, shielding of electrical cable, shielding of rooms wherein it is necessary to minimize interference, etc.
It is known that highly electrically conductive materials provide excellent shielding properties against high frequency interference. Such materials and especially copper have been used to shield out or attenuate high frequency electromagnetic radiations. It is also known that ferromagnetic materials provide excellent shielding properties against low frequency interference. Such materials and especially permalloy (iron-nickel alloy) or iron, 9% silicon, 5% aluminum or iron/ aluminum (up to 10% by weight) or iron with 1 to 3% of silicon ("transformer steel") may be used to shield out low frequency electromagnetic radiations. The ferromagnetic materials, however, vary in their shielding ability, the degree of shielding being based upon the permeability of the material and the width of the hysteresis loop. As the ferromagnetic material becomes saturated, its performance as a shield declines. It is therefore essential to avoid saturation in order to provide optimum shielding performance.
Mu metal, which is a copper-nickel-iron alloy having a high permeability and poor saturation properties, has been widely used by itself to provide electromagnetic shielding. Copper foil alone has also been used as a shield for electromagnetic radiations. Furthermore, separate walls of copper and steel have been used to provide shielding for spatial areas.
Metallurgically bonded copper/stainless steel/copper laminates have been known in the prior art for use in connection with shielding of electromagnetic cables. The stainless steel in such shields is believed to have been used for support strength and rodent protection and not for shielding, 300 series S steels having a so depressed Curie point that they are not magnetic at room temperature. 400 series steels are magnetic, however the presence of chromium depresses the saturation value and widens the hysteresis loop.
U.S. Pat. No. 3,555,169 sets forth a shielding material having inner and outer layers of copper with intermediate layers of stainless steel and low carbon steel.
It has further been known to provide a copper/Invar/copper material for use as heat sink substrates in small printed circuit boards.
Finally, it is known from U.S. Pat. No. 4,156,882 to match materials of different magnetic characteristics in a recording head wherein the layers are insulated from each other with organic materials. This patent relates to the controlled distribution of magnetic lines and not to shielding properties. Insulation between the layers is for the purpose of producing penetration of the whole material, not to the depth of penetration alone.
All of the above noted materials have either not provided the desired shielding properties or have not been used as shielding materials and, even if so used, would not provide shielding properties of the type provided by the shielding materials as disclosed hereinbelow.
As noted above, a further problem in the prior art is that of providing shielded rooms which are free of externally produced interference. A typical design for such room is set forth in U.S. Pat. No. 4,646,046.
It is also known to provide a laminated sheet having exterior layers of copper which sandwich an interior layer of a ferromagnetic alloy taken from the class consisting of nickel-iron compounds having from about 42 to about 80 percent nickel by weight and iron as a substantial portion of the remaining material. Other trace materials, such as but not limited to molybdenum, silicon, manganese can be included. The layers are metallurgically bonded to each other by sintering or the like in well known manner to form the shielding material.